1. Technical Field
The present invention generally relates to packet switched networks and in particular to bi-directional video compression in packet switched networks.
2. Description of the Related Art
In video compression formats, such as in the International Organization for Standardization/International Electro-technical Commission (ISO/IEC) Moving Picture Experts Group (MPEG) video standards, often only the differences between frames are encoded. For example, in a scene in which a person walks past a stationary background, only the moving region is required to be represented (either using motion compensation or as refreshed image data, or as a combination of the two, depending on which representation requires fewer bits to adequately represent the picture). The parts of the scene that are not changing need not be sent repeatedly. Pictures that are used as a reference for predicting other pictures are referred to as “reference pictures”. The three major picture types found in typical video compression designs are Intra coded (I) pictures, Predicted (P) pictures, and Bi-predictive (B) pictures. They are also commonly referred to as I frames, P frames, and B frames. In such designs, the pictures that are coded without prediction from other pictures are called the “I” pictures. Pictures that use prediction from a single reference picture (or a single picture for prediction of each region) are called the “P” pictures. Pictures that use a prediction signal that is formed as a (possibly weighted) average of two reference pictures are called the “B” pictures.
In real-time video streams (such as video telephony, video conferencing and live video streams), the MPEG compression coders/decoders (codecs) are not able to look ahead to make coding decisions. This means that only Intracoded Video Object Plane (I-VOP)/I-frames and P-VOP/P-frames are used to encode the video sequences that use the lowest compression ratios. The B-VOP/B-frames that use bi-directional reference frame information and have the highest compression ratios are not used to make coding/compression decisions. B-VOP/B-frames cannot be used in real-time video transports without introducing delays at the point of video capture and coding/compression.